Refractometers are commonly used in industry to measure the index of refraction in chemical compositions. Once determined, the refraction index of a substance can be used to measure and thus control the concentration of elements within the substance.
Temperature compensated hand-held refractometers are also commonly used to measure refraction indices of vehicle fluids at various temperatures. In industries which maintain a fleet of vehicles, such as buses, trucks or planes, temperature compensated refractometers are able to provide a convenient means for frequently testing the concentration levels of crucial vehicle fluids, such as glycol based anti-freezes and batteries.
Similarly in the chemical, soft-drink or juice industries, it is often necessary to determine the level of sugar in the fluids manufactured. Temperature compensated refractometers having Brix scales for determining percent sugar levels are convenient for such testing.
Refractometer devices previously available are disclosed, for example, in U.S. Pat. Nos. 3,267,795 and 3,329,060. In such prior art refractometers, the testing of fluids is accomplished by removing samples, applying them to the refractometer and reading the results from an internal reticle scale or digital readout. The refractometer is able to compensate for the temperature and concentration of the fluid being measured, and converts the refraction index directly to a protection rating, such as the minimum temperature protection provided by an anti-freeze sample, or the percentage of solids or mixture content in a substance. No further calculation or conversion of the reading is required.
Measuring the concentration or protection levels of manufactured fluids enables rapid determinations to be made as to whether the levels are acceptable or require adjustment. The simplicity of the test procedure also enables measurements to be made by manufacturing or maintenance personnel without special training.
One disadvantage of these prior art devices has been that measurements can only be made in the light, since refraction index measurements are of changes in the light path passing through the substance to be tested. Thus, a light source is required for performing the measurements. Operators of refractometers taking measurements in unlighted or dimly lighted areas during the night have been inconvenienced by having either to move to a well lighted area or to carry an external light source.